The differentiation and function of T lymphocytes is mediated through a series of cell-cell interactions in the thymus and in the peripheral immune system. These interactions are likely to involve several cell surface glycoproteins which appear at different stages in thymocyte development and which are specifically expressed in the T cell lineage. We will begin to study the functions of two such molecules, (L3T4) and T8 (Lyt2), whose genes we have recently isolated. These molecules are expressed on functionally distinct T cell populations, with T4 present primarily on helper cells and T8 on cytotoxic and suppressor cells. In addition, these glycoproteins define the class of major histocompatibility complex (MHC) molecules with which the T cell is restricted to interact. Thus T4+ cells are restricted to recognize antigen and class II MHC molecules, while T8+ cells recognize antigen and class I molecules. We will introduce the mouse L3T4 and Lyt2 genes and the human T4 and T8 genes into T cell clones and into transgenic animals to identify sequences which are required for tissue specific expression of these genes. We will then construct chimeric genes and anti-sense genes which will allow us to ask a number of questions regarding the function of the products in the thymus and in peripheral T cells. Some of these questions include: Do the T4/L3T4 and T8/Lyt2 glycoproteins interact directly with MHC molecules? Do they transduce transmembrane signals in the T cell? Are Lyt2+L3T4+ (double positive) cells, normally found only in the thymus, able to migrate to the periphery in transgenic animals? Is the expression of either of these molecules required for the functional phenotype of the T cell to appear? If double positive cells appear in the peripheral lymph nodes, do they have normal functions, and, if so, what is their pattern of MHC reactivity? We will also study the frequency, tissue distribution, and function of a secreted form of the T8 molecule, which results from variable RNA processing of the T8 mRNA; and we will use methods of subtractive hybridization to isolate the gene encoding the T11 glycoprotein, which is believed to be involved in T cell activation. By bringing a molecular approach to the study of T cells, we hope to begin to answer some of the more interesting unresolved questions concerning their development and function in the peripheral immune system.